Axle shafts of the type used in automotive vehicles are formed of high-carbon steel that is heat treated to increase strength. The treatment comprises inductively heating the steel above about 1750.degree. F. and quickly water quenching. Because of the elongated shape of the shaft, this is conveniently carried out by longitudinally passing the shaft through a station that progressively treats the length of the shaft. The station includes an electrical coil that surrounds an axial zone of the shaft and inductively heats the zone by an alternating electrical current conducted through the coil. Thereafter, the heated zone passes through the quench ring that sprays cooling water onto the steel. As the shaft is passed through the station, it is held in a fixture that includes locators, referred to as centers (centering spindles), that hold the ends of the shaft and rotate the shaft to promote uniform heating and treatment.
A problem in the manufacture of axle shafts is the deviation of the axle axis from a straight line, which is referred to as warpage. Some warpage is found in the shaft prior to heat treatment as a result of the preceding operations. Furthermore, the heat treatment tends to produce warp in the shaft as a result of residual stresses introduced by the rapid heating and cooling sequence. To reduce warpage, the treated shaft is conventionally subjected to a cold straightening operation following heat treatment, which not only is costly, but may introduce cold working stresses into the metal that tend to weaken the shaft.
It is an object of this invention to provide an apparatus for induction heat treating an axle shaft or the like that reduces warpage in the treated shaft. The apparatus not only minimizes warpage that might otherwise result from the heat treatment operation, but also may lessen pre-existing warpage found in the shaft prior to treatment. It is a more particular object of the invention to provide a shaft heat treatment apparatus of the type having a heat treatment station that longitudinally scans a length of a steel shaft to progressively treat the shaft, which apparatus comprises at least one pair of opposing shaft support jaws at an axial zone intermediate the shaft ends that confine the shaft before and after the station to reduce warpage, but disengage the shaft for passage of the zone through the station for unencumbered heat treatment. The apparatus comprises a cam follower operatively connected to the support jaws that engages a cam arrangement at the station, causing the support jaws to bypass the station without colliding therewith, but permitting the jaws to re-engage the shaft prior to the setting of warpage therein.